You create a new field on the Lead and Contact objects in SFDC and two fields are created in Marketo, one mapped to Lead and the other mapped to Contact.
You will see this behavior if the an SFDC field is created on two objects across more than one sync cycle, one before a sync cycle starts and the other one during or after the sync cycle completes. To ensure both the lead and contact show up mapped to the same custom field Marketo, do the following
Disable the global sync in Marketo. (Admin > Salesforce)
Create the field on your desired objects in SFDC. Make sure they both have the same API name.
Re-enable the global sync in Marketo, allowing the field in both objects to sync down to Marketo at the same time.
If you find yourself with multiple fields in Marketo after the sync, contact Marketo Support to have the fields merged or remapped as necessary.